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Temperature Thermistors
Thermistors are electronic devices which sense temperature. As the
temperature increases, the resistance decreases.
10Kohm
thermistors are used to sense outdoor air temperature (OAT), coil
temperature (OCT) and the suction line temperature (OST) located
between the reversing valve and the accumulator. A 50Kohm
thermistor is used to sense discharge temperature (ODT).
Refer to Table 5 and Fig. 35 and 36 for resistance values versus
temperature.
Table 5 – 10K/50Kohm Resistance Values vs Temperature
10Kohm
_C (_F)
TEMPERATURE
25.0 (77.0)
0.0 (32.0)
-28.0 (-18.4)
RESISTANCE (ohms)
10.0 + / --- 2.3%
32.6 + / --- 3.2%
85.5 + / --- 3.4%
50Kohm
125.0 (257.0)
75.0 (167.0)
25.0 (77.0)
1.7 + / --- 1.6%
7.40 + / --- 2.0%
50.0 + / --- 2.3%
THERMISTOR CURVE
90
Failed Thermistor Default Operation
Factory defaults have been provided in the event of failure of
outdoor air thermistor (OAT) and/or outdoor coil thermistor
(OCT).
If the OAT sensor should fail, defrost will be initiated based on coil
temperature and time.
If the OCT sensor should fail, defrost will occur at each time
interval during heating operation, but will terminate after 2
minutes.
If there is a thermistor out--of--range error, defrost will occur at
each time interval during heating operation, but will terminate after
2 minutes.
Count the number of short and long flashes to determine the
appropriate flash code. Table 7 gives possible causes and actions
related to each error.
Outdoor Coil Thermistor
80
RESISTANCE (KOHMS)
S In heating if the outdoor air sensor indicates  35_F ( 19.4_C)
warmer than the coil sensor (or) the outdoor air sensor indicates
 10_F ( 5.6_C) cooler than the coil sensor, the sensors are out
of range.
If the sensors are out of range, the control will flash the appropriate
fault code as shown in Table 7.
The thermistor comparisons are not performed during low ambient
cooling or defrost operation.
The outdoor coil thermistor is a 10Kohm resistor used for multiple
system operations. It provides the coil/liquid line temperature to
the heat pump board and user interface. Low ambient operation,
defrost initiation, defrost termination and assistance with OAT
temperature measurement of some of the functions. The sensor
must be securely mounted to the tube connecting the EXV and
distributor. See Fig. 38 for proper placement. See Table 5 for
proper resistances.
70
60
50
40
30
20
10
0
0
20
40
60
80
TEMPERATURE (DEG. F)
100
120
A91431
Fig. 35 -- 10K Thermistor Resistance Versus Temperature
RESISTANCE (KOHMS)
50K THERMISTOR
450
400
350
300
250
200
150
100
50
0
0
20
40
60
80
100
120
TEMPERATURE (°°F)
A14302
A14022
Fig. 36 -- 50K Thermistor Resistance Versus Temperature
Fig. 37 -- Outdoor Coil Thermistor (OCT) Attachment
(On Distributor Tube)
If the outdoor air or coil thermistor should fail, the control will
flash the appropriate fault code (see Table 7).
IMPORTANT: The outdoor air thermistor, coil thermistor and
suction thermistor should be factory mounted in the final
locations. Check to ensure thermistors are mounted properly
(See Fig. 37, 38, 39 and 40).
OAT Thermistor must be locked in place with
spherical nib end facing towards the front of
the control box
Thermistor Sensor Comparison
The control continuously monitors and compares the outdoor air
temperature sensor and outdoor coil temperature sensor to ensure
proper operating conditions. The comparison is:
S In cooling if the outdoor air sensor indicates  10_F ( 5.6_C)
warmer than the coil sensor (or) the outdoor air sensor indicates
 25_F ( 15_C) cooler than the coil sensor, the sensors are out
of range.
A11142
Fig. 38 -- OAT Thermistor Location (Bottom of Control Box)
Catalog No: II288B--- 05
Manufacturer reserves the right to change, at any time, specifications and designs without notice and without obligations.
18
Replaces: II288B--- 04